Capacitors are used extensively in electronic devices for storing an electric charge. A capacitor includes two conductive plates or electrodes separated by an insulator. The capacitance, or amount of charge held by the capacitor per applied voltage, depends upon the area of the plates, the distance between them, and the dielectric value of the insulator. Capacitors may be formed within a semiconductor device, such as, for example, a dynamic random access memory (DRAM) or an embedded DRAM.
As semiconductor memory devices become more highly integrated, the area occupied by the capacitor of a DRAM storage cell is reduced, thus decreasing the capacitance of the capacitor due to a smaller electrode surface area. However, a relatively large capacitance is desired to prevent loss of stored information. Therefore, it is desirable to reduce the cell dimensions and yet obtain a high capacitance, which achieves both high cell integration and reliable operation.
One technique for increasing the capacitance while maintaining the high integration of the storage cells is directed toward the shape of the capacitor electrodes. In this technique, the polysilicon layer of the capacitor electrodes may have protrusions, fins, cavities, etc., to increase the surface area of the capacitor electrode, thereby increasing its capacitance while maintaining the small area occupied on the substrate surface.
Instead of forming the capacitor on the substrate surface, capacitors are also formed above the substrate, i.e., they are stacked above the substrate. The surface area of the substrate can then be used for forming transistors. U.S. Pat. No. 5,903,493 to Lee discloses a capacitor formed above a tungsten plug. The surface area of the capacitor is increased by etching a trench in the dielectric layer around the tungsten plug. The tungsten plug interfaces with an interconnection line, thus allowing different layers formed above the substrate to be connected.
The trench is patterned by conventional etching or other suitable techniques. The fundamental limit on how far the trench can be etched is determined by how well the tungsten plug is secured within the dielectric layer. Typically, the depth of the trench is limited to about one half the thickness of the dielectric layer. After the trench has been etched, a capacitor is formed above the tungsten plug. Unfortunately, if the trench is etched beyond one half the thickness of the dielectric, the tungsten plug is more likely to become loose and fall out. This physical separation between the tungsten plug and the underlying metal interconnection with interconnect line can cause open circuits to be formed resulting in complete failure of the device or circuit.
A known approach to secure the metal plug uses a metal plug with an anchor portion extending into the interconnect line. This approach uses an additional wet etch, reactive ion etch (RIE) or plasma etch to form an anchor hole in the interconnect line and below the dielectric layer.